Some skills like Item Identification get a little something extra at GM (ID everything in a container in one click). What other skills do something special at 100.0 that doesn't happen at 99.9?
All of them.. you go from a lowly Master that no one respects... to a Grand Master that everyone adores with a .1. Blacksmithing, Carpentry, Tailoring, Inscription, and I think Tinkering.. you get a makers mark at GM.
Lumberjacking dmg bonus 99.9 skill (+8), 100.0 skill (+9) --- Magery + Poisoning = Poison Spell Level (within 3 tiles) - 199.9 = greater, 200.0 = 10% chance for Deadly Magery + Inscription = Magic Reflect Circles - 199.9 = 14.9965, 200.0 = 15 circles worth of reflection